When I go to 'Personal Details' and try to update any of the other information on that page,
I get an error message that "Someone has already chosen that vanity name, please choose a different one!".

This also occurs when trying to edit a user in the ACP, if they have defined a Vanity Name.
The only way I've been able to get around this behavior is by temporarily changing it to gibberish (or something else), making the changes to the personal details, and reverting it back to the original vanity name.

This addon should have Approment feature and name editor feature for mod/admin.
There are some important names which admin can't know how important they are and these names should not be used by others.

This addon should have Approment feature and name editor feature for mod/admin.
There are some important names which admin can't know how important they are and these names should not be used by others.

When I go to 'Personal Details' and try to update any of the other information on that page,
I get an error message that "Someone has already chosen that vanity name, please choose a different one!".

This also occurs when trying to edit a user in the ACP, if they have defined a Vanity Name.
The only way I've been able to get around this behavior is by temporarily changing it to gibberish (or something else), making the changes to the personal details, and reverting it back to the original vanity name.

Blacklist is all names we know to protect. But there are many potential names we don't know but we should protect them.
For example, a member register "support" name. But this name isn't in Blacklist. Then when we need a support system, we can't use it because it was registed by a member. If there is an approment system, when we check and see "support", we don't approve this name to member.

It might also be a nice feature for it to automatically insert the user's username as the default vanity name.
They'd be able to change it, at their discretion, but it'd ensure that at least their username is 'reserved', to an extent.
Though, I imagine this would only impact newly registered users, rather than retroactively affecting existing users.

Looking at what has been posted it seems that this could be done if the user creates a Vanity Name, is this correct?

Also, to make all users have a vanity name the same as their username could you make the field in their settings hidden and set the default of the field to their username. This would save their username as their vanity name automatically. For existing users, you could then run an SQL on the table to update the same for all existing users. The only problem left would be to come up with some way that when a new user registers, the Vanity Name is automatically created from their username when they register.